Resistance network for amplifiers



Oct. 12, 1943.

J. E. MAYNARD 2,331,708

RESISTANCE NETWORK FOR AMPLIFIERS Filed Ma 9, 1942 ITfiveTTt-or: John E.Maynard,

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Patented Get. 12, 1943 2,331,708 RESISTANCE NETWORK FOR AMPLIFIERS JohnE. Maynard, Schenectady, N. Y, assignor to General Electric Company, acorporation of New York Application Mayfi, 1942, Serial No. 442,298

5 Claims.

My invention relates to audio amplifiers and particularly to suchamplifiers in which a portion of the output voltage is supplied to theinput circuit of the amplifier to improve the operation thereof.

One object of my invention is to provide manual volume control meansconnected in the input of such an amplifier in such a way that itsaction does not affect the action of such feed-back voltage supplymeans. v

In amplifiers used in monitoring equipment for broadcast studios, forexample, it frequently happens that the output impedance of theamplifier is adjusted to desired values by properly proportioning thefeed-back voltages. For example, if two components of feed-back voltagebe supplied to the input of the amplifier. one in degenerative phase andvarying as the output voltage, and the other in regenerative phase andvarying as the output current, the amplifier output impedance may bemade to appear as zero if these components are properly related inmagnitude. Similarly, by properly proportioning these components andadjusting their polarities, the amplifier output impedance may be madeto appear large if desired. Once the proper proportion is established,however, it is desirable that it be maintained constant. Hence it isanother object of my invention to provide means whereby adjustment ofthe manual volume control means will not disturb the proportion of thecomponents of the feed-back voltage.

The features of my invention which I believe to be novel are set forthwith particularity in the single figure represents an embodiment of myinvention.

Referring to the drawing, I have illustrated therein a portion of theaudio frequency circuits of a radio receiver to which my invention isapplied. This circuit comprises an audio frequency amplifier l whichamplifies the reproduced audio signal and supplies it to a subsequentaudio power amplifier 2, whose output. in turn, is coupled to outputmeans, for example, the loud A signal voltage supplied across theterminals 3 is impressed .upon the control electrode 5 of the amplifierI. The amplifier i is shown as a triode'having in addition to thecontrol electrode 5, an anode 6 coupled to the cathode l by meansofresistor 8 and bypass capacitor 9; however, any other suitable type ofdischarge device may be used. The output of the anode 6 of amplifier iis coupled to a first control electrode H] of power amplifier 2 by meansof capacitor H and a volume control potentiometer 12. This potentiometercomprises two equal branch resistors i3 and I i and a contact bridgingmember I5. The audio power amplifier 2 may be of the pentode type havingan anode l6, a cathode I1, and control electrodes II], I 8, and I9.Control electrode 18 may be coupled to the cathode, as by means ofbypass capacitor 2!, and connected to a source of potential by means ofresistor 20 to act as the usual screen grid. Control electrode l9 may bedirectly connected to cathode H in a well-known manner to act as theusual suppressor grid. The output of the anode of power amplifier 2 issupplied through an audio transformer 22 to I Potential for the anode ofthe the loud speaker. discharge device 2 is furnished by battery 23through the primary winding of transformer 22.

In order to improve the frequency response characteristics of the audiocircuit described, a combination of regenerative and degenerativefeed-backs is employed. As shown in the drawing, a feed-back voltage indegenerative phase and proportional to the audio output voltage suppliedto the loud speaker 3 is developed across resistor 26 connectedacrossthe secondary winding of transformer 22. Another feed-backvoltage, in regenerative phase and proportional to output current, isdeveloped across the resistor 25 in series with the secondary winding oftransformer 22. The voltages across those portions of these resistorsbetween the tap 26 and ground are supplied to the control electrode inof the audio power amplifier through a feed-back network including thecapacitor 2.! and a high resistance potentiometer having an uppersection 30 and a lower section 3i. If these components are properlyrelated in magnitude and polarity, the amplifier output impedance may bemade to appear as possessing a desired value, either large or small.Thus, by properly adjusting the values of resistors 24 and 25 and thepolarity of the voltages applied to these resistors in a well-knownmanner, this output impedance may be made to appear as zero.

if desired, or may be made to appear as having any larger value. Thischaracteristic is of particular advantage where, instead of beingsupplied to a loud speaker as illustrated in the drawing, the output ofthe amplifier is supplied to some other type of output means, such as afilter or line and cathode ll, which is presented to the feed backvoltages developed in the output circuit, be maintained substantiallyconstant. Such a constant impedance is obtained in my system by means ofthe volume control potentiometer l2. With respect to alternatingcurrents, the discharge device 2 has its cathode connected to oneterminal of resistance 13 and its control electrode l connected throughthe variable resista ance H to a variable point on resistance 13. Theconnection between the variable resistance I4 and the variable point onresistance I 3 is effected by the bridging member l5. Since the twobranch resistances l3 and I4 have substantially equal ohmic values perunit of length, as the contact bridging member I5 is varied, a desiredfraction of the audio voltage from the audio amplifier I is impressedupon control electrode I0 without varying the impedance betweenelectrode I0 and ground. Thus, as the bridging member l5 removes a partof resistance [3 from the circuit between control electrode l0 andcathode II, it inserts into that circuit a part of resistance 14, equalin ohmic value thereto. Consequently the performance of the feed-backnetwork is not altered by adjustment of potentiometer l2.

While I have shown a particular emodiment of my invention, it will ofcourse be understood that I do not wish to be limited thereto sincevarious modifications may be made, and I contemplate by the appendedclaims to cover any such modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In combination, a source of signals, an electron discharge devicehaving an anode, a cathode, and a control electrode, an input circuitconnected between said control electrode and cathode,- an output circuitconnected between said anode and cathode, means for impressing signalsfrom said source between said control electrode and cathode inadjustable intensity, means connected in parallel with said first meansfor impressing a feed-back voltage derived from said output circuitbetween said control electrode and cathode, and means for maintainingthe impedance between said control electrode and cathode presented tosaid feed-back voltage substantially constant as said first means isadjusted to vary the intensity of said signals.

2. The combination in a radio receiver, of an audio amplifier having ananode, a cathode, and a control electrode, an input circuit connectedbetween said control electrode and cathode including a source of signalvoltage to be amplified, an output circuit connected between said anodeand cathode, a transformer in said output circuit having primary andsecondary windings, means connected in parallel with said input circuitfor impressing a feed-back voltage derived from the secondary winding ofsaic transiormer between said control electrocir and .l.7.-

ode, and means for adjusting the intensity of signal voltage amplifierby said amplifier, said means comprising a resistance connected acrosssaid source, the cathode of said discharge device being connected to aterminal of said resistance and said control electrode being connectedthrough a variable resistance to a variable point on said resistance,and means to increase said variable resistance as said variable point isvaried toward said terminal thereby to maintain constant impedancebetween said control electrode and cathode.

3. The combination with a source of signals, of an electron dischargedevice having an anode, a cathode, and acontrol electrode, an inputcircuit connected between said control electrode and cathode, an outputcircuit connected between said anode and cathode, means for impressingsignals from said source between said control electrode and cathode,feed-back means connected in parallel with said first means forimpressing a voltage derived from said output circuit between saidcontrol electrode and cathode, and means for adjusting the intensity ofsignals impressed on said control electrode, said means comprising aresistance connected across said source, the cathode of said dischargedevice being connected to a terminal of said resistance and said controlelectrode being connected through a variable resistance to a variablepoint on said resistance, and means to increase said variable resistanceas said variable point is varied toward said terminal thereby tomaintain constant impedance between said control electrode and cathode.

4. In combination, a source of signals, an electron discharge devicehaving an anode, a cathode, and a control electrode, an input circuitconnected between said control electrode and cathode, an output circuitconnected between said anode and cathode, means for impressing signalsfrom said source between said control electrode and cathode inadjustable intensity, a resistance connected between said controlelectrode and said output circuit and a resistance connected betweensaid cathode and said output circuit thereby to supply voltage derivedfrom said out putput between said control electrode and cathode and inparallel with said signals, and means for maintaining the impedancebetween said control electrode and cathode presented to said voltagesubstantially constant as said first means is adjusted to vary theintensity of said signals.

5. In combination, a first electron discharge device having an inputcircuit and an output circuit for supplying signals, a second electrondischarge device having an anode, a cathode, and a control electrode, aninput circuit connected between said control electrode and cathode, anoutput circuit connected between said anode and cathode, means forimpressing a feed-back voltage derived from said output circuit of saidsecond discharge device between said control electrode and cathode inparallel with the input circuit of said device, a resistance connectedin said output circuit of said first discharge device, said cathodebeing connected to a terminal of said resistance and said controlelectrode being connected through a variable resistance to a variablepoint on said resistance, and means to i rea e aid variable resistanceas said variable point is rant-:1 n said terminal thereby to maintainconstant impesance between said control electrode and cathode.

JOHN E. MAYNARD.

